One example of an electromagnetic signal source is a wireless access point, such as a Wi-Fi base station (Wireless Access Point), which is used to communicate wirelessly with a device by the transmission and reception of electromagnetic radiation in the form of radio waves. Other electromagnetic signal sources may for example include optical (infra-red) communication devices and electromagnetic/wireless beacons of various sorts.
It is sometimes desired to estimate the location of electromagnetic signal sources. In one example, details of the location and other properties of wireless access points (WAPs), cellular telephone masts, and other electromagnetic signal sources are determined. A device operated by a user can then measure properties of electromagnetic signals (such as WAP radio frequency signals) detected at the device, and the location of the user can be calculated with reference to the previously determined location and other data. For example, a smartphone with Wi-Fi capability can determine the identity and signal strength of adjacent WAPs, and a triangulation can be carried out based on the known locations of the WAPs in question in order to determine the location of the smartphone (and hence user). Clearly, the better that the estimation of the location of the signal sources (WAPs) is, the better will be the resulting user location estimates.
In one particular example “war-driving” is used to determine the location of wireless access points (WAPs) within range of a vehicle as it is driven around. A global positioning system (GPS) or similar unit in the vehicle records the location of the vehicle, and signal detection equipment (including for example a highly directional antenna and Wi-Fi interface circuitry) identifies the relative position and other properties of the WAPs. The absolute position of the WAPs can then be determined using the two pieces of information. A similar process can be achieved by “war-walking”, in which scaled-down equipment is carried around by a person to achieve the same effect.
War-driving suffers from limitations in the accuracy with which the position of WAPs and other signal sources can be determined. The propagation of signals is affected by environmental factors, and effects such as multi-path propagation and signal attenuation can become more significant the further away from the signal source that one goes. The necessary distance between vehicles on a road and WAP base stations (typically installed inside buildings away from the road) can lead to significant inaccuracies in the WAP location estimates, and other WAPs may not be detected at all at that distance. These factors can reduce the accuracy of a location service that uses data derived from the war-driving.
War-walking can allow detection equipment to be brought nearer to the WAPs and even inside buildings, but once inside the building the GPS receiver will typically fail, due to the loss of line-of-sight with the GPS satellites.
Therefore both war-driving and war-walking suffer from limitations in the accuracy in the estimate of the location of the WAPs and in some cases do not allow a location to be determined at all due to the failure of the GPS or similar positioning system.